Multilayering of piezoelectric crystals is a well known technique for improving the electrical impedance of a transducer element in comparison to a single layer transducer element having the same thickness as the multilayered transducer element. It has been found that the capacitance of the transducer element increases by N.sup.2 where N is the number of layers forming a transducer element. The increased capacitance of the transducer element results in a decrease in electrical impedance of the transducer element by 1/N.sup.2. This particularly benefits lower frequency arrays as well as transducer elements having small plate areas.
One known method of fabricating a multilayer transducer assembly involves tape casting piezoelectric layers and cofiring electrodes. More particularly, sheets of piezoelectric ceramic are formed from raw materials by tape casting. An internal electrode is formed on a sheet piezoelectric ceramic by screen printing, then another sheet of ceramic is laminated on the internal electrode side of the first sheet. External electrodes are then printed and fired on external sides of the first and second sheets. For example, Saithoh, S. et al., "A Dual Frequency Ultrasonic Probe," Jpn. J. Appl. Phys., vol. 31, suppl. 31-1, pp. 172-4 (1992) describes such a method. Typically connection of the signal electrode formed on the transducer element to leads that will be coupled to an ultrasound system may be made by using a flex circuit or TAB like jumpers or by wirebonding. Connection of the ground electrode formed on the transducer element may be made by using a conductive epoxy that contacts the ground electrode and also makes contact to a secondary connector such as a flex circuit or metal foil.
Several disadvantages exist with fabricating multilayered transducer assemblies by this method. Tape cast piezoelectric material is generally substantially weaker than hot isostatic press piezoelectric material and usually does not survive element definition, i.e., dicing especially fine pitch arrays. Also, because the sheets of piezoelectric ceramic are made from raw materials, in small batches, tape casting does not lend itself to consistency from batch to batch. In addition, tape casting cannot be used to produce sheets having well controlled and/or constant thickness. Moreover, tape casting cannot be used to produce non-planar shapes, such as plano-concave shapes such as that described in U.S. Pat. Nos. 5,415,175 and 5,438,998. Furthermore, the co-fired electrodes are generally thicker, about 4 .mu.m, and have a greater variation in thickness than electrodes deposited by other deposition techniques such as sputter deposition, for example.
Another known method for fabricating multilayered transducer assemblies utilize vias to connect similarly oriented layers. More particularly, multiple holes are punched mechanically or by laser, drilled, or etched into piezoelectric ceramic tape to form the vias on each layer of piezoelectric ceramic. The via holes are filled with a metal paste and the surface electrodes for each layer are deposited by screen printing. Multiple layers of green tape are then superimposed to align the vias to form a multilayer sandwich. The multilayer sandwich is laminated and then finally sintered to form a single package. Metallization is then plated or vacuum deposited on the input pads. For example, U.S. Pat. No. 5,548,564 describes such a method.
Several disadvantages exist with fabricating multilayered transducer assemblies by this method. Forming vias to interconnect similarly oriented layers requires additional steps and thus complicates the fabrication process. In addition, the vias themselves take up valuable real estate in the transducer assembly leaving less real estate for the piezoelectric ceramic. Furthermore, the piezoelectric material may be damaged in the process of creating the vias.
It is thus desirable to provide a multilayer transducer assembly and a method for fabricating the same that allows the use of readily available, high quality piezoelectric material having good mechanical strength. In addition, it is desirable to provide a method for fabricating a multilayered transducer assembly that allows common methods for applying electrodes that provide well controlled and thin electrodes of about 2,000-5,000 .ANG. compared to screen printed and co-fired pastes. Also, it is desirable to provide a method for fabricating multilayered PZT that has precise dimensional control for producing constant thickness, or plano-concave shapes, for example. Furthermore, it is desirable to provide a method for fabricating multilayered transducer assemblies that utilize tools and techniques well known in the ultrasound art.